In U.S. Pat. No. 3,987,703 of Eugene E. Latimer, which patent is owned by the assignee hereof, a hydraulic control system is disclosed having a load-lifting hydraulic motor, a source of pressurized fluid, and a pilot-operated directional control valve for directing fluid for operation of the motor. The system is operative automatically upon failure of the main fluid pressure supply to provide pressurized fluid from the pressure side of the load-supporting motors for emergency pilot operation of the directional control valves.
Jesse L. Field, Jr., in U.S. Pat. No. 3,840,049, which patent is also owned by the assignee hereof, shows a control system for a fluid motor wherein the direction control valve includes a spool and means for selectively shifting the spool in response to fluid pressure signals from a manually operated pilot valve. The direction control valve includes make-up valve means for opening a bypass passage between the motor and the fluid return line when necessary to prevent cavitation of the motor such as due to an overrunning condition from the external load.
Josef Distler shows, in U.S. Pat. No. 3,766,944, a pilot controlled fluid flow regulating valve. The pilot valve has a body provided with inlet means connected to a pump or other suitable source of pressurized fluid, and port means connected to the chamber in the housing of the regulating valve. A valve member of the pilot valve is movable to control the flow of fluid between the inlet means and the port means. A control spring is mounted in the body of the pilot valve and is deformable to displace the valve member against the opposition of fluid pressure acting on the surface of the valve member.
Franz Forster et al., in U.S. Pat. No. 3,698,415, show a pressure regulating valve having a control member shiftable in a cylindrical bore of the bore housing by a manually operable lever. A plurality of springs are provided for interconnecting the control member and the valve body. One of the springs comes into play only after the control member has been displaced at least a predetermined amount.